How to route the outgoing mails in Linux

In our concern we r using exchange server as mail server its ip address is 10.0.0.8 and we have one linux system i installed two network cards in it ,
one for lan ip 10.0.0.25 another one for isp public ip 203.101.70.254 , through this i can connect internet from this public ip

via i can access internet , now how can i transfer or route the internal mails in exchange server to linux system to send externally.